Experimental test and mechanism analysis of soil crust erosion resistance of rammed earth Great Wall in rainy season

Rammed earth is a kind of cleaning material, widely used in all kinds of buildings in the world. The Great Wall of ancient China is a typical world cultural site built from rammed earth. The rammed earth Great Wall of Shanhaiguan is close to Bohai Bay, which has suffered from long-term erosion by rain, causing a series of problems such as soil loss, collapse and gully flushing. The protection materials of the rammed earth site have always puzzled scholars. However, during the rainy season, it was found that some of the walls at Xiaowan Gouge and Nantuzhuang Gouge in the Shanhaiguan Great Wall had unwashed traces, the soil surface of the walls was intact, and the anti-erosion ability of the walls was significantly higher than that of other places. In order to explore the reasons for its strong anti-erosion ability in the natural state of rammed earth wall, guide the protection of rammed earth Great Wall, and carry out different experimental tests to explore its anti-erosion reasons and internal mechanisms. Firstly, the characteristics of rammed soil were understood through the composition test of rammed soil, and the indoor and outdoor erosion test was carried out to determine that the anti-erosion reason was the protection of gray-green soil crust. The property and composition of soil crust were determined through the immersion test and genome sequencing. Finally, the protection mechanism of soil crust was analyzed by scanning electron microscopy.

derivatives, resulting in ion exchange and flocculation that cement soil particles together 17,18 .All the above are manual soil surface treatment to improve soil anti-erosion performance.However, after investigation, no manual treatment has been carried out here, and the local erosion resistance of this place is naturally improved.The discovery that the protection of natural materials has better adaptability than traditional restoration methods has aroused our great interest.We first need to determine what this grayish-green thin layer of material is that forms under natural conditions.as well as its resistance to erosion, and reveal its mechanism.so that it can be applied to the conservation of rammed earth sites.
Soil crust is a natural phenomenon that exists widely on the soil surface in arid desert areas all over the world 19,20 .Soil crust can be divided into biological soil crust (BSC) 21,22 and physical soil crust (PSC) (J.et al.,  1984; Mcintyre, 1958  23,24 according to its morphology and genesis.BSC are mainly composed of cyanobacteria, lichens and algae, as well as bacteria, mosses and fungi 25 .PSC usually forms through the consolidation of soil materials due to rain effects or through the redistribution and accumulation of fine particles during surface runoff 24,26,27 .Soil crust can significantly affect ecosystem processes 28 , Including desert system ecology [29][30][31] , soil hydrology 32 , soil physicochemical properties 33,34 and the carbon cycle 35 .Previous studies have found that biological soil crust occurs mostly in arid and semi-arid areas, and rarely in areas with abundant rainy season.At the same time, the soil properties of the place are mainly sandy soil, and less appear in the cohesive soil of rammed earth.Therefore, we need to identify the soil crust characteristics of the gray-green thin layer structure found at the rammed earth site in Bohai Bay.
In order to explore the reasons for the high local anti-erosion ability of the rammed earth Great Wall in Shanhaishan during the rainy season, different testing methods were adopted to explore its formation reasons and internal mechanism.Firstly, the characteristics of rammed earth in the Great Wall are tested.Then through the field erosion test and indoor erosion test, it is confirmed that the gray-green soil crust on the surface of rammed soil improves the erosion resistance of the soil.After that, the biological characteristics of soil crust were determined by immersion test and genome test.Finally, the internal mechanism of soil crust resistance to erosion was analyzed.The research results will provide important guidance for the protection and restoration of the Great Wall site, avoid the disaster of erosion and erosion, and provide promoting significance for the construction of the Great Wall cultural park in China.

Physical geography
The Shanhaiguan Great Wall is located in Shanhaiguan District, Qinhuangdao City, Hebei Province, China (Fig. 1).It was built in the 14th year of Hongwu during the Ming Dynasty (AD 1381).As an important military defense system in ancient China against northern aggression, it was deemed a World Cultural Heritage site in December 1987 8 .
The Great Wall of Shanhaiguan is adjacent to Bohai Bay and experiences a temperate continental monsoon climate with four distinct seasons.According to the data of the Qinhuangdao Meteorological Bureau, the average annual temperature of Qinhuangdao is 10.5 °C, the highest monthly average temperature, 24.4-25.3°C, occurs in July, and the lowest monthly average temperature, -8.4 to -4.7 °C, occurs in January.The extreme maximum temperature in the area is 40.3 °C, and the extreme minimum temperature is -29.2 °C.The average annual precipitation is 645.3-693.5 mm, the maximum annual precipitation is 1273.5 mm, and the annual minimum precipitation is 332 mm.Precipitation is mostly concentrated from June to August, with an average precipitation of 399.9-460.6 mm, accounting for 68% of the average annual precipitation.From November to February, the precipitation is the lowest, with an average precipitation of 18.78 mm, accounting for 3.18% of the average annual precipitation.The maximum daily rainfall is 291.0 mm (1984.8.10), and the maximum accumulated rainfall is 614 mm (July 20, 2016 4:00 a.m.-July 21, 2016 8:00 a.m.).The annual number of sunshine hours ranges from 2403-3113 h.

History of the Great Wall of Shanhaiguan, China
Since the Great Wall was built a long time ago, it has suffered various degrees of damage due to natural, war and man-made effects over hundreds of years.According to historical records, the Great Wall of Shanhaiguan has been renovated many times in history; it was repaired during the Qing Dynasty, and it was repaired many times after the founding of New China.The foundation of the Great Wall of Shanhaiguan adopts a strip stone foundation, the surface of the wall is covered with sintered gray bricks, and the inner wall core is rammed earth.The cross-section of the Great Wall is trapezoidal, the wall height is approximately 8 m, the top width is approximately 3-5 m, and the bottom width is approximately 6-8 m (Fig. 1c).In some areas, the wall tiles on both sides have fallen off, leaving only the rammed earth wall core.The rammed earth wall core is mainly silty clay, with gravel in some sections.After the rammed earth wall core loses the protection of the wall tiles, the soil is eroded by the rainwater, forming a gulley.After sun exposure, the wet wall dries, and the water evaporates, resulting in dry cracks.Repeated rainfall and sun exposure results in shrinkage-expansion cycles and salt deposition for the rammed earth wall 10 .This leads to a series of problems, such as soil deterioration and collapse (Fig. 2).

Characteristics of the local erosion resistance of the rammed earth wall
Rammed earth has unique properties as the core of the Great Wall.According to historical records, the rammed earth core of the Great Wall was built together with the wall bricks during the masonry process of the Great Wall.The soil was made of silty clay mixed with gravel 36 .Using a rammer, multiple people lifted and then dropped it to ram the soil to form a dense structure.Some reinforced sections were mixed with lime, glutinous rice pulp, etc., to enhance the strength of the wall.Soil materials were often obtained from nearby areas to facilitate their use.The rammed wall core soil has greater strength and can stand upright even if the protection of the wall tiles is lost.
Field observations show that the Xiaowan Gap and Nantuzhuang Gap of the Great Wall have the same characteristics as the erosion resistance wall parts; that is, a gray-green thin-layer structure is formed on and covered the soil surface.The structural layer is relatively dense, and the thin-layer structure in some sections can be peeled off by prying it with a knife.The thickness of the thin-layer soil skin measured with a Vernier caliper is approximately 1-2.5 mm.Previous researchers called this thin-layer soil skin the crust layer 23,[37][38][39][40] .After peeling

Rammed earth materials
The soil samples used in this experiment were obtained from the Great Wall of Shanhaiguan.The rammed earth Great Wall surface crust layer is sample A1 (gray-green thin-layer structure), the powder layer of the lower crust layer is sample A2, the wall parent soil is sample A3, the weathered rock block near the wall is sample A4, the rammed earth from the parent soil is sample A5, Sample sampling locations are shown in Table 1.Powder B1 and B2 made from A5 rammed soil sample.Six to ten pieces of each sample were taken for parallel testing.

Rammed earth Great Wall parent soil composition
The parent soil of the A3 sample rammed earth Great Wall is silty clay, which is yellowish-brown.The soil texture is hard.The test method is in accordance with the requirements of "Standards for Geotechnical Test Methods" (GB/T50123-2019).Its physical and mechanical properties are shown in Table 2.
The fine particle content of samples A1, A2 and A3 was tested, and the results were shown in Fig. 4. In the figure, the left side is the percentage of each grain group, and the right side is the cumulative percentage.As can be seen from Fig. 4, the content of clay particles in the layer A1 is the highest, followed by A3 in the parent soil layer, and A2 in the silt layer is the least.The silt content in the silted layer A2 is the highest, followed by the mother soil layer A3, and the layer A1 is the least.

Identification of glutinous rice syrup and lime mixture
According to historical records, glutinous rice pulp and lime were added to some sections of the Great Wall of Shanhaiguan during the construction period.In this way, the mechanical strength of the rammed earth Great Wall was increased, coupled with the protection of the outer brick to improve the overall performance of the Great Wall.The main component of glutinous rice pulp is starch.By adding iodine to the rammed earth aqueous solution, it is determined whether glutinous rice pulp was incorporated.The test method is shown in Table 3. From the comparison of Fig. 5a and b, it can be seen that the B1 specimen did not show a color reaction, while the B2 specimen did show a color reaction after artificially adding glutinous rice slurry into the rammed soil.
Under long-term action, CaCO 3 will be produced if lime is added to the rammed earth in the wall.Diluted hydrochloric acid with a concentration of 0.5 mol/L was added to crust layer sample A1, pulverized layer sample A2, parent soil layer A3, and no foaming occurred.
The CaCO 3 content of A1, A2 and A3 samples was identified.The test method was carried out in accordance with the requirements of the Standard for Geotechnical Test Methods (GB/T50123-2019), and the results are shown in Fig. 5.It can be seen from Fig. 6 that the CaCO 3 contents of the three samples of A1, A2, and A3 are low, with the mass percent contents of 0.25%, 0.67%, and 0.52%.
According to the ancient literature and a large amount of archaeological data, the formula of the rammed earth of the Great Wall at Shanhaiguan does contain ingredients such as glutinous rice paste and lime.However, after a long time of weathering, chemical action and decomposition, not much of the original ingredients such

Gray-green crust layer
Sample A1

Pulverized layer Sample A2
Parent soil layer Sample A3

Salt identification
The salt content of layer A1, powder layer A2 and parent soil layer A3 were tested respectively.The test method is in accordance with the requirements of "Standards for Geotechnical Test Methods" (GB/T50123-2019), and the results are shown in Fig. 7.In terms of the total content of salts, the crust layer had the lowest, then the parent soil layer, and the pulverized layer had the highest.
Table 1.Test samples.The process of A5 sample fabrication is as follows: the mother soil sample of the wall taken was dried and processed, and the dried soil sample was crushed with a wooden hammer and passed through a 2 mm sieve.A homemade 7 × 7 × 7 cm mold was used as the compacted sample mold.According to the measured optimal moisture content of water, according to 16% moisture content of water with a spray bottle, evenly sprayed on the surface of the soil body, into a sealed bag, sealed for 24 h standby.The weight of 2.5 kg tamping hammer was used to tamp in three layers, each layer was filled with 213 g of soil, and each layer was tamped with 5 strikes, and finally compacted to a uniform height with 2000psi pressure under a hydraulic press.The size of the specimen was 7 × 7 × 6.7 cm, weighing 639 g, and the specimen was cured under natural condition for 28 days after the specimen was made.

XRD test
XRD tests can be used for qualitative and quantitative phase analysis of powder samples, which can be used to analyze the mineral composition and proportion of the layer sample A1, the layer sample A2, the wall mother soil sample A3, and the weathered rock block A4 near the wall.The test was carried out using the Smart Lab diffractometer of Nikaku, Japan, with a working environment of 40kv-200 mA and an angular speed of 2 degrees per minute.
The mineral composition of soil is shown in Fig. 8.As a whole, there is no significant difference in the mineral composition of the soil, mainly quartz (SiO2), microcline (K [AlSi3O8]), albite (Na2O•Al2O3•6SiO2).The composition of layer A1 mineral is basically the same as that of weathered rock A4, indicating that the three minerals with strong peak value in the diffraction pattern of layer A1 are primary minerals, while the peak value of secondary minerals is lower.The content of quartz in soil is higher than that of weathered rock, indicating that quartz is less easily decomposed by weathering in soil.

Field erosion test of rammed earth Great Wall
At the rammed earth wall site, three sites were selected as experimental sites.At erosion site 1, the wall was completely covered by a gray-green thin-layer structure, denoted as E1.At erosion site 2, part of the city wall was covered by a gray-green thin-layer structure, denoted as E2.At erosion site 3, there was an ordinary section of wall that had not yet been covered, denoted as E3.Erosion experiments were carried out using a pressure pump with a water spray rate of 3.365 mL/min.The sprinkler was 30 cm away from the city wall surface, the pressure of the outlet pipe was 0.257 MPa, and the rain flushing time was 5 min/48 h.Plastic buckets were used to collect the washed soil and water mixture.After standing for 48 h, the water in the bucket was extracted and dried, and the quality of the washed soil was measured.

Laboratory erosion test
The original sample D1 of the gray-green thin-layer structure, the original sample D2 of the parent soil layer, and the rammed soil sample D3 made from the parent soil were used for the laboratory rainfall erosion test, which was used to compare and test the erosion resistance of the undisturbed samples of the gray-green thinlayer structure with the parent soil and rammed soil samples.The laboratory erosion test system is shown in Fig. 9.The pressure pump was used at 3.365 mL/min.The distance between the spray port and the soil sample was 30 cm, the pressure of the outlet pipe was 0.257 MPa, and the rainfall flushing time was 5 min.The erosion process was recorded with a camera.After the erosion test, the soil-water mixture was dried to measure the quality of the scoured soil.

Immersion test
The gray-green thin-layer structure on the erosion resistance wall surface found in the field was removed with a knife and was cultured in deionized water and BG11 medium prepared in laboratory 41 .The composition of BG11 (Culture medium for soil crust) is shown in Table 4.The surface changes were observed after 7 days.

SEM test
The gray-green thin-layer structure sample A1 and the parent soil layer A3 were selected for drying treatment and polished into 3 mm × 3 mm × 5 mm cubes with a knife and sandpaper for SEM observation.The test instrument used was a VEGA3 (LM) fully automated tungsten filament scanning electron microscope produced by Czech TESCAN.The experimental magnification was 500-20,000 times.

Field erosion test results
The 5-min field erosion test results are shown in Fig. 11.It can be seen from the figure that for E1, after the water droplets contacted the crust surface, the water flowed along the surface of the gray-green thin-layer structure, the water body was clear, and the crust layer surface did not change within 5 min.For E2, water infiltration Table 5. Sampling locations.had not yet formed a crust layer.After the parent soil layer was in direct contact with the water droplets, the water body infiltrated, and the phenomenon of mud flow and slumping occurred at 1 min.The soil body collapsed at 2 min, and the mud block flowed together with the water flow at 3 min, causing serious soil erosion.After 5 min of erosion, the soil mass was dried and weighed as follows: 32.77 g (E1), 895.10 g (E2), and 2926.11g (E3).It can be seen that the gray-green thin-layer structure in the field has good erosion resistance.

Laboratory erosion test results
The laboratory 5-min erosion test results are shown in Fig. 12.As seen from the figure, the water droplets of the original sample of the D1 cortex layer flowed along the surface after encountering water, the overall stability was better, and only the erosion marks appeared at the edges.This erosion mark was caused by the untreated edge of the soil sample, and the surface was wet after washing.The undisturbed parent soil sample D2 was changed by water infiltration and wetting processes in the first 2 min after encountering water.At 3 min, the edge soil block fell, and at 3.5 min, the large block slipped and collapsed.After 4 min, the sample was washed by rain and mud flows.The rammed earth sample D3 was changed by a water infiltration process in the first 3 min after encountering water.Small gullies appeared in the local edges and corners, and the edges and corners slumped at 3.5 min.After 4 min and 10 s, the remaining edges and corners began to slump.Finally, after 5 min, all four edges and corners collapsed to form a pentagonal structure.Before erosion, the masses of samples D1, D2, and D3 were 108.87 g, 912.65 g, and 572.62 g, respectively.After 5 min of erosion, the resulting sample masses were 103.72 g, 663.11 g, and 392.95 g, accounting for 95.27%, 72.66%, and 68.62%, respectively.The overall erosion resistance results show an order of D1 > D2 > D3.It can be seen that the gray-green thin-layer structure also has a very good erosion resistance performance.

Immersion test results
The gray-green thin-layer structure samples (Fig. 13a) were placed in distilled water and BG11 culture medium.Figure 13b shows that the gray-green thin-layer structure sample did not disintegrate after being wetted in water, indicating that it has a certain bond strength and disintegration resistance.It can be seen from Fig. 13c and d that after 7 d of incubating the gray-green thin-layer structure sample, bubbles formed in the water, and biological growth occurred on the surface.The gray-green thin-layer structure samples were biological crust, and biological crusts produce air bubbles during photosynthesis in water.Figure 13d shows that algae and moss grew on the surface of the biological crust.This shows that biological crust is a complex system that involves a process of continuous evolution and growth by the joint action of algae and moss.When water and nutrients are sufficient and light and temperature are suitable for growth, and the biological crusts will transition from algal crusts to moss crusts.

Genome sequencing results
Through genome sequencing of the gray-green thin-layer structure, it was observed that the gray-green thinlayer structure is a biological soil crust layer.The test results are shown in Table 6.The soil crust layer of the organism contains a total of 28 phyla, 65 classes, and 140 orders, and the composition mainly comprises cyanobacteria and alphaproteobacteria, accounting for 60% (Fig. 14).This shows that the soil crust layer is a composite soil crust layer dominated by algae and supplemented by fungi.

SEM microstructure
Figure 15 shows the SEM results of A1 junction cortex with magnification of 500-2000× for comparative observation.Under the scanning electron microscope, it can be seen clearly from Fig. 15b that the algae present filamentous, overlapping with each other and covering the surface of the soil particles.As can be seen from Fig. 15c, the polysaccharide mucous secreted by algae covers the surface of the soil particles in a flake form and wants

Erosion protection mechanism of biological crusts on the rammed earth Great Wall
The experimental results show that algal biological crusts can effectively prevent rainwater erosion.The erosion resistance mechanism of algal crust is explained as follows: First, from a macroscopic perspective, algal crusts form a protective layer on the wall surface, which can effectively resist and alleviate the impact of water droplets on soil particles, reduce soil splash erosion and infiltration, and increase surface runoff 42,43 .On the other hand, after rainfall, algae can quickly absorb residual water and reduce soil damage caused by wetting.Second, from a microscopic point of view, algae secrete polysaccharides, lipids and proteins under photosynthesis that have a cohesive effect, which can effectively bind soil particles 44,45 , as shown in Fig. 17.On the other hand, algal filaments and mycelia wrap and wind among the soil particles, which can firmly bind soil particles and form a special multilayer network structure with polysaccharide mucinous secretions, thus enhancing the soil surface stability.
The findings of this study not only have applications in the protection of the Great Wall against rainwater erosion but also offer a viable method for the conservation of similar rammed earth heritage sites.Biological soil crusts, as part of the local biotic community, exhibit excellent adaptability and environmental friendliness, capable of sustaining themselves on the surfaces of rammed earth sites over the long term.This represents a distinct advantage over traditional engineering protective measures.

Further problems to be solved
Biological crusts have a positive effect on the erosion resistance of cultural relics in soil sites, which has been confirmed by field observations and the above experiments.The mitigation mechanism for rain erosion has also been explained.However, which algae are the dominant algal species in biological crusts, how algae and fungi play a protective role together, and the inhibitory effect of algae crusts on the growth of plants on the surface of the Great Wall also require further in-depth research.Our future research endeavors involve extracting, isolating, and on-site inoculation demonstrations of the Great Wall.The Great Wall of Shanhaiguan is adjacent to the Bohai Bay, which has a temperate continental monsoon climate and four distinct seasons.An important factor affecting its durability is the freeze-thaw process.In the future, we will carry out research on the impact of freezing and thawing on the durability of rammed earth sites.

Conclusions
This work reveals the reason of local erosion resistance of rammed earth Great Wall site.The following conclusions are obtained from the material composition and erosion test of rammed earth: 1.The improvement of local anti-erosion ability of Xiaowan Gap Great Wall is not due to the formation of calcification and precipitation on the soil surface and the change of mineral composition in the wall.It is caused by the formation of biological soil crust on the soil surface.

Figure 1 .
Figure 1.The Great Wall of Shanhaiguan, China: (a) Shanhaiguan Great Wall location map (This figure were generated used by the software Bigemap, the URL link is http:// www.bigem ap.com/), (b) the first pass in the world, (c) Nantuzhuang Great Wall gap, (d) Xiaowan Great Wall gap.

Figure 2 .
Figure 2. Types of damage to the Great Wall.
green thin layer structure on the surface of Shanhaiguan rammed earth Great Wall A2 Obtained by scraping the A1 layer with a knife A3 Sample of the interior of a rammed earth wall in its original state A4 Rock samples around the Great Wall of rammed earth A5 The rammed earth from the parent soil

Figure 7 .
Figure 7. Salt concentration in the soil.

Figure 10 .Figure 11 .
Figure 10.Sampling points of the crust samples of the Xiaowan Gap rammed earth Great Wall.

Figure 13 .
Figure 13.Growth status of soil crust samples after culture.

Figure 15 .
Figure 15.SEM photo of the crust layer.

Figure 16 .Figure 17 .
Figure 16.SEM photo of the parent soil layer.
as glutinous rice paste and lime are left.Through calcium carbonate content testing experiments, it was found that the calcium carbonate content at different depths was very low.Therefore, it can be concluded that biological soil crusts are the dominant factor inducing localized erosion resistance rather than calcium carbonate deposition effects.

Table 2 .
Physical and mechanical property parameters of the A3 sample rammed earth.ω denotes the water content; ρ is the density; G s represents the specific gravity; S r is the saturation; ω L and ω P are the liquid limit and plastic limit, respectively; I P corresponds to the plasticity index; ω op is the optimum water content.

Table 4 .
BG11 medium ratio.Genome sequencingGenome sequencing was performed on the surface layer of 0-3 mm undisturbed gray-green thin-layer structure samples from the upper, middle, lower and surrounding surfaces of the Great Wall.The sampling locations are shown in Table5and Fig.10.The test process was DNA extraction → PCR amplification → amplicon library construction → on-machine sequencing.